Recognize variadic generics type variables (PEP 646).

[tristanlatr]

Fixes #629.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (0636f0d) 92.49% compared to head (eecd664) 92.49%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #708 +/- ## ======================================= Coverage 92.49% 92.49% ======================================= Files 47 47 Lines 8103 8103 Branches 1935 1935 ======================================= Hits 7495 7495 Misses 354 354 Partials 254 254 ``` | [Impacted Files](https://app.codecov.io/gh/twisted/pydoctor/pull/708?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=twisted) | Coverage Δ | | |---|---|---| | [pydoctor/astbuilder.py](https://app.codecov.io/gh/twisted/pydoctor/pull/708?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=twisted#diff-cHlkb2N0b3IvYXN0YnVpbGRlci5weQ==) | `95.99% <100.00%> (ø)` | |

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[tristanlatr]

This should be trivial to review